Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus includes an illumination system including a field faceted mirror including a plurality of field facets and configured to receive radiation from a radiation source and form a plurality of images of the radiation source on corresponding pupil facets of a pupil faceted mirror. Each of the field facets is configured to provide an illumination slit at a level of a patterning device. The illumination slits are summed together at the level of the patterning device to illuminate the patterning device. First blades are configured to block radiation from a radiation source and each first blade is selectively actuable to cover a portion of a selectable number of field facets. The field faceted mirror further comprises partial field facets, the partial field facets being configured to produce a partial illumination slit at the level of the patterning device, and the pupil faceted mirror further includes pupil facets corresponding to the partial field facets. The partial field facets are configured to produce an illumination slit that is summed with the summed illumination slits of the field facets and/or correct for non-uniformity in the summed illumination slits of the field facets. Second blades are selectively actuable to cover a portion of a selectable number of partial field facets.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S.application Ser. No. 10/784,895, filed Feb. 24, 2004 and entitled“LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD,” which is acontinuation-in-part of U.S. application Ser. No. 10/388,766, filed Mar.17, 2003 and entitled “LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURINGMETHOD,” now U.S. Pat. No. 6,771,352, issued Aug. 3, 2004, which claimspriority from European Application No. 02251933.4, filed Mar. 18, 2002,the entire contents of each of which are herein incorporated byreference. This application incorporates by reference U.S. applicationSer. Nos. 10/379,999, filed Mar. 6, 2003 and entitled “MASK FOR USE INLITHOGRAPHY, METHOD OF MAKING A MASK, LITHOGRAPHIC APPARATUS, AND DEVICEMANUFACTURING METHOD,” and 10/157,033, filed May 30, 2002 and entitled“LITHOGRAPHIC APPARATUS, DEVICE MANUFACTURING METHOD, DEVICEMANUFACTURED THEREBY, CONTROL SYSTEM, COMPUTER PROGRAM, AND COMPUTERPRODUCT.” This application incorporates by reference U.S. Pat. No.6,583,855, issued Jun. 24, 2003 and entitled “LITHOGRAPHIC APPARATUS,DEVICE MANUFACTURING METHOD, AND DEVICE MANUFACTURED THEREBY.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic projection apparatus anddevice manufacturing method.

2. Description of the Related Art

The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). An example of such a patterning device is amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support willgenerally be a mask table, which ensures that the mask can be held at adesired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

Another example of a patterning device is a programmable mirror array.One example of such an array is a matrix-addressable surface having aviscoelastic control layer and a reflective surface. The basic principlebehind such an apparatus is that, for example, addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate filter, the undiffracted light can be filtered out of thereflected beam, leaving only the diffracted light behind. In thismanner, the beam becomes patterned according to the addressing patternof the matrix-addressable surface. An alternative embodiment of aprogrammable mirror array employs a matrix arrangement of tiny mirrors,each of which can be individually tilted about an axis by applying asuitable localized electric field, or by employing piezoelectricactuators. Once again, the mirrors are matrix-addressable, such thataddressed mirrors will reflect an incoming radiation beam in a differentdirection to unaddressed mirrors. In this manner, the reflected beam ispatterned according to the addressing pattern of the matrix-addressablemirrors. The required matrix addressing can be performed using suitableelectronics. In both of the situations described above, the patterningdevice can include one or more programmable mirror arrays. Moreinformation on mirror arrays as here referred to can be seen, forexample, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCTpublications WO 98/38597 and WO 98/33096. In the case of a programmablemirror array, the support may be embodied as a frame or table, forexample, which may be fixed or movable as required.

Another example of a patterning device is a programmable LCD array. Anexample of such a construction is given in U.S. Pat. No. 5,229,872. Asabove, the support in this case may be embodied as a frame or table, forexample, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device as setforth above.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (IC's). In such a case, thepatterning device may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. including one or more dies) on a substrate (silicon wafer)that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once. Such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus, commonlyreferred to as a step-and-scan apparatus, each target portion isirradiated by progressively scanning the mask pattern under theradiation beam in a given reference direction (the “scanning” direction)while synchronously scanning the substrate table parallel oranti-parallel to this direction. Since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be seen, for example, fromU.S. Pat. No. 6,046,792.

In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. It is important to ensure that the overlay (juxtaposition) of thevarious stacked layers is as accurate as possible. For this purpose, asmall reference mark is provided at one or more positions on the wafer,thus defining the origin of a coordinate system on the wafer. Usingoptical and electronic devices in combination with the substrate holderpositioning device (referred to hereinafter as “alignment system”), thismark can then be relocated each time a new layer has to be juxtaposed onan existing layer, and can be used as an alignment reference.Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the radiation beam, and such components may also bereferred to below, collectively or singularly, as a “lens”. Further, thelithographic apparatus may be of a type having two or more substratetables (and/or two or more mask tables). In such “multiple stage”devices the additional tables may be used in parallel or preparatorysteps may be carried out on one or more tables while one or more othertables are being used for exposures. Dual stage lithographic apparatusare described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.

Correct imaging of a pattern generated by a patterning device in alithographic projection apparatus requires correct illumination of thepatterning device. In particular, it is important that the intensity ofillumination proximal the plane of the pattern, as generated by thepatterning device, or proximal planes conjugate to the plane of thepattern, be uniform over the area of the exposure field. Also, it isgenerally required that the patterning device can be illuminated withoff-axis illumination in a variety of modes such as, for example,annular, quadrupole or dipole illumination, to improve resolution. Theuse of such illumination modes is disclosed, for example, in U.S. Pat.No. 6,671,035. The illumination modes are obtained, for example, byproviding a corresponding predetermined intensity distribution in apupil of the illumination system.

To meet the above-mentioned requirements, the illumination system of alithographic projection system is generally quite complex. A typicalillumination system might include: shutters and attenuators configuredto control the intensity of the beam output by the source, which mightbe a high pressure Hg lamp or an excimer laser; a beam shaping elementsuch as, for example, a beam expander for use with an excimer laserradiation beam configured to lower the radiation beam divergence; azoomable-axicon pair and a zoom lens configured to set the illuminationmode and parameters (collectively referred to as a zoom-axicon); anintegrator, such as a quartz rod, configured to make the intensitydistribution of the beam more uniform; masking blades configured todefine the illumination area; and imaging optics configured to projectan image of the exit of the integrator onto the patterning device. Forsimplicity, the plane of the pattern generated by the patterning device,and planes conjugate to this plane in the illumination system and theprojection system may be referred to hereinafter as “image” planes.

The illumination system may also include elements configured to correctnon-uniformities in the illumination beam at or near image planes. Forexample, the illumination system may include diffractive opticalelements to improve the match of the radiation beam cross-sectionproximal the entrance face of the integrator rod with the shape of theentrance face. A diffractive optical element typically includes an arrayof microlenses, which may be Fresnel lenses or Fresnel zone plates.Improving the match alleviates the problem of field dependentlithographic errors occurring in the patterned layer. The matching mayhereinafter be referred to as “filling” of the integrator entrance face.A diffractive optical element may also be positioned, for example, infront of a beam shaping element, such as a zoom-axicon, to transform theangular distribution of radiation provided by an excimer laser beam intoa predetermined angular distribution of radiation to generate a desiredillumination mode. Illumination systems as discussed above aredisclosed, for example, in U.S. Pat. Nos. 5,675,401 and 6,285,443.

The illumination system may also include, for example, a filterpartially transmissive to radiation of the radiation beam with apredetermined spatial distribution of transmittance, immediately beforethe plane of the pattern, to reduce spatial intensity variations.

The illumination systems discussed above still suffer from variousproblems, however. In particular, various elements that are used,especially diffractive optical elements and quartz-rod integrators, canintroduce an anomaly of intensity distribution in a plane perpendicularto the optical axis of the radiation system or the projection system.For example, in a plane proximal a pupil of the radiation system or theprojection system, either the beam cross-section may be ellipticalrather than circular, or the beam intensity distribution within the beamcross-section may, for example, be elliptically symmetric rather thancircularly symmetric. Both types of errors are referred to as“ellipticity of the radiation beam” or simply as “ellipticity error,”and typically lead to specific lithographic errors in the patternedlayers. In particular, a patterned feature occurring in directionsparallel to both the X and Y direction may exhibit, in the presence ofellipticity of the beam, different sizes upon exposure and processing.Such a lithographic error is usually referred to as H-V difference.Also, a diffractive optical element used to improve filling of theintegrator is generally only optimum for one setting of the zoom-axicon.For other settings, the integrator entrance face may be under-filled(the beam cross-section is smaller than the integrator entrance face),leading to substantial field dependent H-V difference. For othersettings, the integrator entrance face may also be over-filled, leadingto energy wastage. Additionally, 157 nm excimer lasers and other excimerlasers tend to have large divergence differences in X and Y directionswhich cannot be completely resolved using beam expander lenses whilekeeping the shape of the beam within acceptable dimensions.

Ellipticity errors in the radiation beam may also be caused bysubsequent elements, such as the mask and elements of the projectionsystem. Current lithographic apparatus do not correct or compensate forellipticity errors introduced into the radiation beam by elementssubsequent to the illumination system.

In a lithographic apparatus, it is important that illumination of thepatterning device is uniform in field and angle distribution and, forillumination modes such as dipole and quadrupole illumination, all polesare equal. To achieve this, an integrator is provided in theillumination system. In a lithographic apparatus using UV or DUVexposure radiation the integrator may take the form of a quartz rod or aso-called fly's eye lens. A fly's eye lens is a lens built up of a largenumber of smaller lenses in an array which creates a correspondinglylarge number of images of the source in a pupil plane of theillumination system. These images act as a virtual, or secondary,source. However, when using EUV exposure radiation the illuminationsystem must be constructed from mirrors because there is no knownmaterial suitable for forming a refractive optical element for EUVradiation. In such an illumination system, a functional equivalent to afly's eye lens can be provided using faceted mirrors, for instance asdescribed in U.S. Pat. Nos. 6,195,201, 6,198,793 and 6,452,661. Thesedocuments describe a first, or field, faceted mirror which focuses aplurality of images, one per facet, on a second, or pupil, facetedmirror which directs the light to appropriately fill the pupil of theprojection system. It is known from UV and DUV lithography that imagingof different types of mask patterns can be improved by controlling theillumination settings, e.g. the filling ratio of the pupil of theprojection system (commonly referred to as a) or the provision ofspecial illumination modes such as annular, dipole or quadrupoleillumination. More information on illumination settings can be obtainedfrom U.S. Pat. No. 6,452,662 and U.S. Pat. No. 6,671,035.

In an EUV lithographic apparatus with a fly's eye integrator, theseillumination settings can be controlled by selectively blocking certainof the pupil facets. However, because the source position and size oneach facet is not exactly known and not stable, it is necessary to blockoff whole facets at a time, rather than partial facets. Thus, onlyrelatively coarse control of illumination settings is possible. Also, toprovide an annular illumination setting it is necessary to obscure theinnermost pupil facets and when positioning a masking blade over aninner facet it is difficult to avoid partially obscuring one or more ofthe outer facets.

Referring to FIGS. 14 a and 14 b, a pupil plane includes a plurality ofoverlapping pupils. The overlapping pupils produce a field plane. Asshown in FIGS. 14 a and 14 b, a square field plane is formed by fouroverlapping cones of radiation in the pupil plane. In the pupil plane,the pupil shape can be corrected for all field positions withtransmission filters. The pupil distribution cannot be influenced overthe field in a controlled manner. Only a pupil correction that isconstant over the field can be applied. In the field plane, theuniformity profile can be changed for all angles with transmissionfilters. Transmission filters can only change the transmission for allangles.

In order to make field dependent corrections to the pupil distribution,the correction must take place between the pupil plane and the fieldplane. The difficulty is that the pupil of one field point overlaps withthe pupil of an adjacent field point. In an intermediate plane, the leftportion of a pupil corresponding to a right field point overlaps withthe right portion of a pupil corresponding to a left field point.Accordingly, it is not possible to correct one portion of the pupil forone field point without affecting the pupil for another field point.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a lithographicapparatus includes a radiation system configured to provide a beam ofradiation. The radiation system includes an illumination system. Asupport is configured to support a patterning device and the patterningdevice is configured to pattern the beam of radiation according to adesired pattern. A substrate table is configured to hold a substrate.The apparatus also includes a projection system configured to projectthe patterned beam of radiation onto a target portion of the substrate.The illumination system includes a field faceted mirror including aplurality of field facets and configured to receive radiation from aradiation source and form a plurality of images of the radiation sourceon corresponding pupil facets of a pupil faceted mirror. Each of thefield facets is configured to provide an illumination slit at a level ofthe patterning device. The illumination slits are summed together at thelevel of the patterning device to illuminate the patterning device.First blades are configured to block radiation from the radiationsource, each first blade being selectively actuable to cover a portionof a selectable number of field facets.

According to another embodiment of the present invention, a devicemanufacturing method includes providing a beam of radiation using aradiation system, the radiation system including an illumination system;patterning the beam of radiation with a pattern in its cross-section;projecting the patterned beam of radiation onto a target portion of alayer of radiation-sensitive material at least partially covering asubstrate, wherein the illumination system comprises a field facetedmirror comprising a plurality of field facets and configured to receiveradiation from a radiation source and form a plurality of images of theradiation source on corresponding pupil facets of a pupil facetedmirror, each of the field facets being configured to provide anillumination slit at a level of a patterning device configured topattern the beam of radiation, the illumination slits being summedtogether at the level of the patterning device to illuminate thepatterning device, and first blades configured to block radiation fromthe radiation source, each first blade being selectively actuable tocover a portion of a selectable number of field facets.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andextreme ultra-violet radiation (EUV), e.g. having a wavelength in therange 5-20 μm, especially around 13 nm, as well as particle beams, suchas ion beams or electron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich:

FIG. 1 is a schematic illustration of a lithographic projectionapparatus according to an exemplary embodiment of the present invention;

FIG. 2 is a schematic illustration of a lithographic projectionapparatus according to an exemplary embodiment of the present invention;

FIG. 3 a is a schematic illustration of an illumination system accordingto an exemplary embodiment of the present invention;

FIG. 3 b is a schematic illustration of an illumination system accordingto another exemplary embodiment of the present invention;

FIG. 4 is a schematic illustration of an ellipticity correction deviceaccording to an exemplary embodiment of the present invention;

FIG. 5 is a further schematic illustration of the device of FIG. 4;

FIG. 6 is a schematic illustration of a portion of the device of FIGS. 4and 5;

FIGS. 7 a and 7 b are perspective and plan view schematic illustrations,respectively, of a radiation beam;

FIG. 8 is a schematic illustration of a device according to anotherexemplary embodiment of the present invention;

FIGS. 9 a-9 c are schematic illustrations of the operation of a step andscan lithographic apparatus according to an exemplary embodiment of thepresent invention;

FIG. 10 is a schematic illustration of an effective or total pupil spotas seen by a wafer in a step and scan lithographic apparatus accordingto FIGS. 9 a-9 c;

FIG. 11 is a schematic illustration of an illumination system accordingto an exemplary embodiment of the present invention;

FIG. 12 a is a schematic illustration of an illumination systemaccording to an yet another exemplary embodiment of the presentinvention;

FIG. 12 b is a schematic illustration of the illumination system of FIG.12 a in a different configuration;

FIG. 13 a is a schematic illustration of an illumination systemaccording to a still further exemplary embodiment of the presentinvention;

FIG. 13 b is a schematic illustration of the illumination system of FIG.13 a in a different configuration;

FIGS. 14 a and 14 b are schematic illustrations of a radiation beam ofan illumination system of a step and scan lithographic apparatus;

FIG. 15 schematically depicts a field faceted mirror in relation to thefield distribution after the radiation source and collector module;

FIGS. 16 a and 16 b schematically depict the summing of field facets;

FIG. 17 schematically depicts the blocking of field facets;

FIG. 18 schematically depicts the images of the distribution at eachfield facet and the combined distribution of the distributions over thefield facets;

FIG. 19 schematically depicts schematically depicts the images of thedistribution at each field facet and each partial field facet and thecombined distribution of the distributions over the field facets and thepartial field facets;

FIG. 20 schematically depicts the blocking of partial field facets;

FIGS. 21A-21C schematically depict the combined distributions over theblocked, partially blocked and unblocked partial field facets as shownin FIG. 21; and

FIG. 22 schematically depicts the intensity for an uncorrected fieldfaceted mirror, the intensity corrected for field faceted mirroraccording to the prior art, and the intensity corrected for a fieldfaceted mirror according to the present invention, including partialfield facets.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatus 1according to an embodiment of the invention. The apparatus 1 includes abase plate BP. A illumination system is configured to supply a beam PBof radiation (e.g. EUV radiation). A radiation source SO is configuredto provide radiation to the illumination system IL. The source SO andthe apparatus 1 may be separate, for example when the source is a plasmadischarge source. In such case, the source SO is not considered to formpart of the apparatus 1 and the radiation beam is generally passed fromthe source LA to the illumination system IL with the aid of a radiationcollector including, for example, suitable collecting mirrors and/or aspectral purity filter. In other cases, the source SO may be integralwith the apparatus 1, for example when the source SO is a mercury lamp.The present invention encompasses both of these scenarios. The source SOand the illumination system IL may be referred to as a radiation system.

A first object (mask) table MT provided with a mask holder is configuredto hold a mask MA (e.g. a reticle), and is connected to a firstpositioning device PM configured to accurately position the mask withrespect to a projection system or lens PL. A second object (substrate)table WT provided with a substrate holder is configured to hold asubstrate W (e.g. a resist-coated silicon wafer), and is connected to asecond positioning device PW configured to accurately position thesubstrate with respect to the projection system PL. The projectionsystem or lens PL (e.g. a mirror group) is configured to image anirradiated portion of the mask MA onto a target portion C (e.g.comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a reflective type (i.e. has areflective mask). However, in general, it may also be of a transmissivetype, for example with a transmissive mask. Alternatively, the apparatusmay employ another kind of patterning device, such as a programmablemirror array of a type as referred to above.

The source SO (e.g. a discharge or laser-produced plasma source)produces radiation. The radiation is fed into the illumination systemIL, either directly or after having traversed a conditioning device,such as a beam expander Ex, for example. The illumination system IL mayinclude an adjusting device AM that sets the outer and/or inner radialextent (commonly referred to as σ-outer and σ-inner, respectively) ofthe angular intensity distribution in the radiation beam. In addition,it will generally include various other components, such as anintegrator IN and a condenser CO. In this way, the beam of radiation PBimpinging on the mask MA has a desired uniformity and intensitydistribution in its cross-section.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning device PW andinterferometer IF, the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of the beamPB. Similarly, the first positioning device PM can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library, or during ascan. In general, movement of the object tables MT, WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step and scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed. The mask MA and the substrateW may be aligned using mask alignment marks M₁, M₂ and substratealignment marks P₁, P₂.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at once, i.e. a single “flash,” onto atarget portion C. The substrate table WT is then shifted in the X and/orY directions so that a different target portion C can be irradiated bythe beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash.” Instead, themask table MT is movable in a given direction (the “scan direction”,e.g., the Y direction) with a speed v, so that the radiation beam PB iscaused to scan over a mask image. Concurrently, the substrate table WTis simultaneously moved in the same or opposite direction at a speedV=Mv, in which M is the magnification of the lens PL (typically, M=¼ or⅕). In this manner, a relatively large target portion C can be exposed,without having to compromise on resolution.

FIG. 2 shows the projection apparatus 1 according to an exemplaryembodiment including the illumination system IL, the source SO, and theprojection system PL. The source SO includes a source collector moduleor radiation unit 3. The illumination system IL includes an illuminationoptics unit 4. A radiation system 2 includes the source-collector moduleor radiation unit 3 and the illumination optics unit 4. The radiationunit 3 is provided with the radiation source SO which may be formed by adischarge plasma. An EUV radiation source may employ a gas or vapor,such as Xe gas or Li vapor in which a very hot plasma may be created toemit radiation in the EUV range of the electromagnetic spectrum. Thevery hot plasma is created by causing a partially ionized plasma of anelectrical discharge to collapse onto the optical axis 0. The radiationemitted by radiation source SO is passed from a source chamber 7 into acollector chamber 8 via a gas barrier structure or “foil trap” 9. Thegas barrier structure 9 includes a channel structure such as, forexample, described in U.S. Pat. No. 6,359,969. The collector chamber 8includes a radiation collector 10, which may be formed by a grazingincidence collector. Radiation passed by the collector 10 is reflectedoff a grating spectral filter 11 to be focused in a virtual source point12 at an aperture in the collector chamber 8. From the collector chamber8, the radiation beam is directed to the illumination optics unit 4.

The illumination optics unit 4 includes a field faceted mirror 13including a plurality of field facets (not shown) and a pupil facetmirror 14 including a plurality of pupil facets 120 (FIGS. 4 and 5). Inthe present embodiment, three mirrors 15 a, 15 b, 15 c magnify and shapethe beam 16 before it impinges on the mask on the mask table MT. Thepath of the beam 16 is shown in more detail in FIGS. 3 a and 3 b. Themirror 15 a may preferably be a toroidal mirror, the mirror 15 b maypreferably be a spherical mirror, and the mirror 15 c may preferably bea grazing incidence mirror. A patterned beam 17 is formed which isimaged in projection system PL via reflective elements 18, 19 onto thewafer on the substrate table WT. More elements than shown may generallybe present in illumination optics unit 4 and the projection system PL.

Referring to FIG. 4, there is a position along the beam trajectorybetween the pupil faceted mirror 14 and the mask where the cross-sectionof the beam 16 is substantially rectangular. The size of the beamcross-section at this position corresponds to the size of the fieldfacets of mirror 13. The cross-section of the beam at this position isblurred by a convolution with the pupil spot distribution and is thusnot in focus with respect to the field plane. As shown in FIGS. 3 a and3 b, the position is after the pupil faceted mirror 14 and in front ofone of the mirrors 15 a, 15 b, or 15 c, and preferably relatively closeto the field plane compared to the pupil plane. It should be appreciatedthat an ellipticity correction device 20 may be positioned other than asshown in the exemplary embodiments of FIGS. 3 a and 3 b.

As shown in FIG. 4, the position is in front of one of the mirrors 15 a,15 b, or 15 c and the actual size of the field plane at the position isshown by solid lines 100. The size of the field plane before convolutionwith the pupil spots 120 is shown by dashed lines 110. The cross-sectionof the beam at the position is substantially rectangular. Pupil spots120, including integrated pupil spots 125, are distributed throughoutthe cross section of the beam at the position. The device 20, includinga plurality of adjustable blades 22, 24 is provided at the position. Theblades 22, 24 are adjustable by linear actuators 23, 25 (FIG. 5) to beinserted into the beam in the Y direction. The position of the device 20is schematically illustrated in FIG. 3 a as prior to the mirror 15 a ina direction of propagation of the beam, but as discussed above thedevice 20 may be positioned in front of either mirror 15 b or 15 c. Thedevice 20 may be positioned anywhere between the pupil faceted mirror 14and the mask, such as shown in FIG. 3 b for example, but is preferablypositioned relatively close to the field plane in comparison to thepupil plane.

Referring to FIGS. 4 and 5, the positions of the blades 22, 24 areadjustable by the linear actuators 23, 25 to be insertable into the beam16 in the Y direction so that part of the pupil spots 120, 125 arecovered. The blades 22, 24 are adjusted so that only those pupil spots120, 125 that correspond to positions at the edge of the cross sectionof the beam 16 in the Y direction are covered. The amount of radiationincident under an angle in the Y direction, i.e. the “vertical light,”is decreased in those pupil spots 120, 125 that are covered, wholly orpartially. Thus, when a scan is performed the integrated vertical lightis less than the “horizontal light,” i.e., the radiation incident underan angle in the X direction. The device 20 corrects or compensates theellipticity value (the ratio of the vertical light to the horizontallight) of the beam at the position by taking into account theellipticity error(s) that may be introduced into the beam by subsequentelements of the illumination system, the mask, and/or the projectionsystem. The ellipticity value can be corrected or compensated bydecreasing the amount of vertical light only. By absorbing or blockingvertical light at selected positions in the X direction, the totalamount of light decreases and the uniformity, i.e. intensity variationover the cross section of the beam, is corrected or compensated. Thevariation may be corrected by an apparatus such as the one disclosed inU.S. Pat. No. 6,404,499, for example.

Referring to FIG. 5, the device 20 includes partially opaque blades 22and solid blades 24 connected to respective linear actuators 23 and 25so as to be selectively and incrementally insertable into the beam inthe Y direction. The blades 22 and 24 may also be reflective. The bladesmay have a reflection profile that varies from completely reflecting tofully absorbing. The blades 22 and 24 are constructed as a plurality offingers 26 and 27, one of each per column of field or pupil facets thatare selectively and incrementally extendible to cover ones of the fieldor pupil facets. However, it should be appreciated that pairs or groupsof fingers may be grouped together.

The partially opaque fingers may be constructed, for example, as a gridof rods, bars or wires or by forming apertures in a suitable pattern ina solid plate. A partially opaque finger 26 formed with a plurality ofrods 29 is shown in FIG. 6.

In general, for other shapes and arrangements of pupil spots, the blades22 and 24 may be arranged differently. As an alternative to adjustableblades, it is possible to provide a plurality of blades, partiallyopaque, solid and/or reflecting, corresponding to desired illuminationsettings and selectively interpose these into the beam 16 as desired.Such fixed blades may be formed as plates with appropriate openings andpartially opaque (apertured) areas and may be held in a magazine orcarousel to be inserted into the beam.

By providing multiple partially opaque blades with different blockingratios, smaller increments of the pupil filling ratio σ can be provided.For example, with two appropriately aligned partially opaque blades, oneblocking 25% and one blocking 50%, quarter steps can be provided. A stepis one row or column on the field or pupil facet mirror. For example, byextending the 25% blade one more step than a solid masking blade aquarter step is provided. Extending the 50% blade one step more providesa half step and both the 25% and 50% blade extended one more stepprovides a three quarter step.

Referring to FIGS. 7 a and 7 b, the field plane and the pupil plane fora beam in a step and scan lithographic apparatus are schematicallyillustrated. In an intermediate plane, there is a field-likedistribution in the X (non-scanning) direction and a pupil-likedistribution in the Y (scanning) direction. As in the exemplaryembodiment discussed above, it is possible to place a filter at theedges of the intermediate plane in the Y direction. To properly performa pupil compensation or correction, the compensation or correction mustbe done relatively far from the field plane. The correction orcompensation is preferably performed relatively far from the reticle. Ina step and scan apparatus, the beam cross section has a relatively smalldimension in the Y (scanning) direction and a relatively largerdimension in the X (non-scanning) direction. The beam cross section isthus pupil-like in the scanning (Y) direction relatively close to thefield plane and field-like in the non-scanning (X) direction. The pupilmay thus be corrected in the scanning direction by inserting filtershaving predetermined absorbing patterns into the intermediate plane inthe scanning (Y) direction.

Referring to FIG. 8, blades or plates 28 having predeterminedtransmissibility/absorbing patterns are insertable in the Y directioninto the edges of the intermediate plane to correct the ellipticityvalue. The blades 28 are insertable and adjustable by linear actuators21, similar to the embodiment described above. Thetransmissibility/absorbing pattern of the blades 28 may be formed toprovide a predetermined field dependence to the device 20A. As shown,the pattern of the blades 28 varies in the non-scanning (X) directionand thus the correction or compensation can vary in the non-scanningdirection.

Referring to FIGS. 9 a-c, in a step and scan lithographic apparatus 1,the pupil P as seen on the substrate W changes during a scan. Theilluminated area IA is defined as a sharp image of a quartz rodintegrator (discussed in more detail below). FIG. 9 a schematicallyillustrates the pupil P1 as seen by the substrate W immediately afterthe beginning of a scan. FIG. 9 b schematically illustrates the pupil P2as seen by the substrate W in the middle of a scan. FIG. 9 cschematically illustrates the pupil P3 as seen by the substrate Wimmediately prior to the end of a scan. Although the step and scanapparatus illustrated in FIGS. 9 a-c uses a transmissive mask, it shouldbe appreciated that the present invention also contemplates the use ofreflective masks.

Referring to FIG. 10, the sum of all the static pupils P1-P3 seen by thesubstrate results in the effective or scanning pupil P. Although threestatic pupils P1-P3 are schematically illustrated, it should beappreciated that during a scan a significantly larger number of staticpupils are summed to produce the effective or scanning pupil.

Referring to FIG. 11, an illumination system IL according to anotherexemplary embodiment of the present invention includes a radiationsource SO, for example a high pressure Hg lamp provided with anelliptical reflector to collect the output radiation or a laser. Twoshutters are provided to control the output of the lamp: a safetyshutter 111 held open by a coil 111 a and arranged to closeautomatically if any of the panels of a casing of the lithographicapparatus are opened. A rotary shutter 112 is driven by a motor 112 afor each exposure. A second rotary shutter 113 is driven by a motor 113a and a light attenuating filter in the shutter aperture may be providedfor low-dose exposures.

Beam shaping is principally performed by an axicon 115 and zoom lens116, which are adjustable optical elements driven by respective servosystems 115 a, 116 a. These components are referred to collectively asthe zoom-axicon. The axicon 115 includes a concave conical lens andcomplementary convex conical lens whose separation is adjustable.Setting the two elements of the axicon 115 together providesconventional circular illumination, while moving them apart creates anannular illumination distribution.

The zoom lens 16 determines the size of the beam or the outer radius ofan annular illumination mode. A pupil shaping device (not shown) may beinserted, for instance, in the exit pupil 122 of the zoom-axicon moduleto provide quadrupole or other illumination modes. Coupling opticscouple the light from the zoom-axicon into an integrator IN.

The integrator IN includes, for example two elongate quartz rods 117joined at a right-angle prism 117 a, the hypotenuse surface of which ispartially silvered to allow a small, known proportion of the beam energythrough to an energy sensor ES. The beam undergoes multiple internalreflections in the quartz rods 117 so that, looking back through it,there is seen a plurality of spaced apart virtual sources, thus eveningout the intensity distribution of the beam. It should be appreciatedthat a fly's eye lens could be used as the integrator.

After the exit of the integrator IN the device 20A is positioned tocorrect or compensate for the ellipticity value of the illumination areaIA, i.e. the area of the reticle to be illuminated. The condensingoptics CO form an objective lens for imaging the masking orifice, via anintermediate pupil 123, on the mask MA. A folding mirror 220 is includedfor convenient location of the illumination system in the apparatus.

According to another exemplary embodiment of the present invention, thepupil filling ratio σ may be controlled by providing adjustable,reflective coated blades in the illumination system. This exemplaryembodiment is useful in lithographic apparatus using EUV radiation asblades which absorb radiation, such as those disclosed in U.S.application Ser. No. 10/388,766, will experience a significant heat loadwhich may adversely affect the performance of the faceted mirrors usedin EUV illumination systems and other optical elements.

Referring to FIGS. 12 a and 12 b, the illumination system includes blademirrors BM selectively insertable or positionable in front of facets ofthe field faceted mirror 13. The blade mirrors BM are coated to bereflective. The coating may include a surface roughness so that thereflected radiation is scattered. Such a coating is disclosed in U.S.application Ser. No. 10/379,999, incorporated herein by reference. Thecoating may also include a phase changing structure so that thereflected radiation from individual blade mirrors cancel each other.Such a coating including a phase changing structure is disclosed in U.S.application Ser. No. 10/379,999.

The radiation reflected by the blade mirrors BM is directed to a beamdump BD which may be cooled to prevent adverse effects on theperformance of the apparatus.

Referring to FIGS. 13 a and 13 b, an illumination system according toanother exemplary embodiment of the present invention includes blademirrors BM selectively insertable or positionable in front of facets ofthe pupil faceted mirror 14. The blade mirrors BM are coated so as to bereflective. The coating may include a surface roughness such that thereflected radiation is scattered or may include a phase changingstructure so that the reflected radiation from individual blade mirrorscancel each other, as discussed above. The radiation reflected by theblade mirrors BM is directed to a beam dump BD which may be cooled toprevent adverse effects on the performance of the apparatus. As thepupil facets are smaller than the field facets, the pupil spotsreflected by the blade mirrors of FIGS. 13 a and 13 b are smaller thanthe field facets reflected by the blade mirrors of FIGS. 12 a and 12 b,thus a smaller beam dump may be used.

Although the embodiments shown in FIGS. 12 a and 12 b and 13 a and 13 bdescribe the blade mirrors as inserted or positioned in front of thefield faceted mirror and the pupil faceted mirror, respectively, itshould be appreciated that blade mirrors may be placed in front of boththe field faceted mirror and the pupil faceted mirror. It should also beappreciated that the various exemplary embodiments may be used incombination. For example, the devices shown in FIGS. 3 a-6, 8 and 11 maybe used in combination with the embodiments shown in FIGS. 12 a-13 a.

Referring to FIGS. 15-17, the field facet mirror 13 includes a pluralityof field facets 13 i. As shown in FIG. 15, the area delineated by thedashed line indicates the area illuminated by the source SO. In thisembodiment, each field facet 13 i creates a full illumination slit atthe mask level. The illumination slits produced by each of the fieldfacets 13 i are summed together. Each of the field facet-images carriesa part of the pupil. By summing all the illumination slits the completepupil is obtained. This leads to the following impact on the pupil dueto uniformity correction: by blocking a given field facet, or part of agiven field facet, the part of the pupil which is carried by theblocked, or partially blocked, field facet is missing in the blockedarea. This impact can be reduced by selecting those facets which carrythe center part of the pupil and/or by selecting the facets that carryopposite parts of the pupil. The pupil effect can be used to vary thepupil parameters across the illuminated slit, as discussed below withreference to FIGS. 16 a and 16 b. In this embodiment, as shown in FIG.15, there are 2×6 (i.e. 12) groups of field facets 13 i and a total of122 field facets 13 i. Each field facet 13 i may be 55 mm×3 mm. Itshould be appreciated that other configurations, sizes, and total fieldfacets may be used.

Referring to FIG. 16 a, all of the field facets, four in this example,are unobscured. Each field facet carries a part of the pupil as shown inthe circle below each facet. The sum of all the facets results in anilluminated slit with at each point of the slit the same pupil: the sumof the pupils. Referring to FIG. 16 b, field facet 1 and field facet 4are partly obscured. In the sum of the field facets the pupil variesacross the field in a controlled way.

Referring to FIG. 17, each of the field facets 13 i can be covered,partially or completely, in the Y (scanning) direction by blade mirrorsBM. As each field facet 13 i creates a full illumination slit at masklevel, changing the effective width (Y) of each field facet 13 i bypositioning the blade mirrors BM in the X (non-scanning) directionallows the integrated dose to be made more uniform. In this embodiment,as there are 122 field facets 13 i, each field facet 13 i can add (i.e.correct) 1/122 (0.82%) intensity-wise to the integrated dose and add0.5/122 (0.41%) uniformity-wise to the integrated dose. By moving theblade mirrors BM over a group of N field facets 13 i, any profile can bemade with a strength of N*0.41% uniformity. As each field facet 13 icontributes only 0.41% to the total integrated intensity, a usefulcorrection range will require the adjustment of several field facets 13i at once. However, as the field facets 13 i are grouped and aligned,multiple field facets 13 i can be covered by a single blade mirror BM.

The embodiment of FIGS. 15-17 may be provided by, for example, anarrangement as shown in FIGS. 12 a and 12 b. In the embodiments shown inFIGS. 12 a, 12 b, and 15-17, as the uniformity correction is performedat the field plane, the effects or changes to the pupil plane can bereduced because the pupil plane is generated by the pupil faceted mirrorwhich follows the field faceted mirror. By separating the uniformitycorrection and pupil plane generation, problems, such as pole balance,ellipticiy, etc., can be avoided.

In illumination systems including fly eye integrators, the field facetsare illuminated completely to obtain a uniform illumination profile. Inorder to obtain this uniform illumination profile, field facets thatwould be partially illuminated are shielded, for example by blademirrors, although it should be appreciated that other devices may beused to shield the partially illuminated field facets. Residualnon-uniformity of the illumination profile may be corrected by filteringaway radiation locally. This may reduce intensity, and thus reducethroughput of the lithographic projection apparatus.

The inventors have discovered that field facets that represent only partof the illumination slit (i.e. partial field facets) may be used withthe field facets that represent the full illumination slit (i.e. fullfield facets) if the partial field facets are complementary, i.e.together they create an entire, uniform illumination slit, or thepartial field facets compensate for non-uniformity of the illuminationslit that is formed by the full field facets. In addition, the inventorshave discovered that partially shielding the partial field facets in anautomatically adjustable manner provides dynamic uniformity correction.

The use of partial field facets allows more of the radiation from thesource to be used, thereby increasing intensity and throughput.Correcting uniformity by adding radiation via the partial field facetsreduces radiation loss, thus increasing intensity and throughput.

Referring to FIG. 18, the images of the distribution at each individualfield facet are imaged in the same location, i.e. all field facet imagesoverlap. The combined distribution is the sum of the distributions overthe individual field facets. Ideally, the combined distribution isuniform. However, in reality the combined distribution is non-uniform,due to factors such as source angular distribution, coatings on variouselements of the apparatus, etc.

Referring to FIG. 19, partial field facets 13 ip are provided in areasaround the field faceted mirror 13. The partial field facets 13 ip donot provide a full illumination slit at the mask level as the fieldfacets 13 i of the field faceted mirror 13 do. However, as shown in FIG.19, the partial field facets 13 ip can be designed to add to the sum,i.e. combined distribution, of the field facets 13 i of the fieldfaceted mirror 13 to provide a uniform distribution. By addingpreviously unused radiation to the combined distribution, the intensityof the illumination is increased and the throughput is increased.

The partial facets 13 ip may be designed to be complementary, i.e., toprovide a full illumination slit at reticle level. The partial facets 13ip may be designed to correct for uniformity errors created by the fieldfacets 13 i of the field faceted mirror 13, or other components of thelithographic apparatus. Each partial field facet 13 ip requires its owncorresponding pupil facet. The partial field facets 13 ip may containall parts of the field, including the side and center. Pairs ofidentical partial field facets may be used and they may be symmetricalin the pupil, which maintains the telecentricity of the system at zero.

As also discussed above, another problem of known illumination systemsis that uniformity problems are corrected by filtering out excessradiation. While this filtering provides more uniform illumination, italso reduces the intensity and the throughput.

Referring to FIG. 20, the loss of intensity caused by filtering tocorrect non-uniformity may be compensated for by the use of partialfield facets 13 ip or parts of partial field facets 13 ip. Blade mirrorsBM may be used to block, entirely or partially, selected partial fieldfacets 13 ip to compensate for non-uniformity of the intensity.Referring to FIGS. 21A-21C, the use of blade mirrors BM to block partialfield facets 13 ip, or parts of partial field facets 13 ip, allows forcompensation of the uniformity of the intensity, for example by addingradiation in the middle of the illumination slit as shown in thisembodiment. Selection of partial field facets may be done at the fieldfacets or at the corresponding pupil facets. Partial selection ofpartial field facets may be done at the field facets.

Referring to FIGS. 21A-21C, the present invention provides uniformitycorrection without a loss of intensity as compared to conventionaluniformity correction techniques, such as filtering excess radiation.The use of partial field facets to add previously unused radiation andthe use of blade mirrors to selectively block partial field facets, orparts of partial field facets, permits uniformity correction withoutloss of intensity, thus increasing throughput of the lithographicapparatus. Although the blade mirrors are shown in FIGS. 21A-21C asconfigured to block a partial field facet, or a portion of a partialfield facet, it should be appreciated that the blade mirrors may also beconfigured to block the partial field facets, or portions of the partialfield facets, as shown in FIG. 17.

It should be appreciated that the partial field facets may be blocked,either completely or partially, by devices other than blade mirrors. Forexample, blades similar to reticle masking (REMA) blades may be used toprovide a hard transition. As another example, moving wedge shapedblades or bath-tub shaped blades may be used to provide grey tones. Asstill another example, the blades may be as described in relation toFIG. 5, or contain coatings as described throughout.

Referring to FIG. 22, the present invention provides uniformitycorrection without a loss in intensity as experienced with currentcorrection techniques.

While embodiments of the invention have been described above, it will beappreciated that the invention may be practiced otherwise than asdescribed. The description is not intended to limit the invention.

1. A lithographic projection apparatus, comprising: a radiation systemconfigured to provide a beam of radiation, the radiation systemincluding an illumination system; a support configured to support apatterning device, the patterning device configured to pattern the beamof radiation according to a desired pattern; a substrate tableconfigured to hold a substrate; and a projection system configured toproject the patterned beam of radiation onto a target portion of thesubstrate, wherein the illumination system comprises a field facetedmirror comprising a plurality of field facets and configured to receiveradiation from a radiation source and form a plurality of images of theradiation source on corresponding pupil facets of a pupil facetedmirror, each of the field facets being configured to provide anillumination slit at a level of the patterning device, the illuminationslits being summed together at the level of the patterning device toilluminate the patterning device; and first blades configured to blockradiation from the radiation source, each first blade being selectivelyactuable to cover a portion of a selectable number of field facets. 2.An apparatus according to claim 1, wherein the field faceted mirrorfurther comprises partial field facets, the partial field facets beingconfigured to produce a partial illumination slit at the level of thepatterning device, and the pupil faceted mirror further comprises pupilfacets corresponding to the partial field facets.
 3. An apparatusaccording to claim 2, wherein the partial field facets are configured toproduce an illumination slit at the level of the patterning device thatis summed with the summed illumination slits of the field facets.
 4. Anapparatus according to claim 2, wherein the partial field facets areconfigured to correct for non-uniformity in the summed illuminationslits of the field facets.
 5. An apparatus according to claim 2, whereinthe partial field facets comprise a pair of identical partial fieldfacets configured to be symmetrical in a pupil plane of the pupilfaceted mirror.
 6. An apparatus according to claim 2, further comprisingsecond blades configured to block radiation from the radiation source,each second blade being selectively actuable to cover a portion of aselectable number of partial field facets.
 7. A device manufacturingmethod, comprising: providing a beam of radiation using a radiationsystem, the radiation system including an illumination system;patterning the beam of radiation with a pattern in its cross-section;projecting the patterned beam of radiation onto a target portion of alayer of radiation-sensitive material at least partially covering asubstrate, wherein the illumination system comprises a field facetedmirror comprising a plurality of field facets and configured to receiveradiation from a radiation source and form a plurality of images of theradiation source on corresponding pupil facets of a pupil facetedmirror, each of the field facets being configured to provide anillumination slit at a level of a patterning device configured topattern the beam of radiation, the illumination slits being summedtogether at the level of the patterning device to illuminate thepatterning device, and first blades configured to block radiation fromthe radiation source, each first blade being selectively actuable tocover a portion of a selectable number of field facets.
 8. A methodaccording to claim 7, wherein the field faceted mirror further comprisespartial field facets, the partial field facets being configured toproduce a partial illumination slit at the level of the patterningdevice, and the pupil faceted mirror further comprises pupil facetscorresponding to the partial field facets.
 9. A method according toclaim 8, wherein the partial field facets are configured to produce anillumination slit at the level of the patterning device that is summedwith the summed illumination slits of the field facets.
 10. A methodaccording to claim 8, wherein the partial field facets are configured tocorrect for non-uniformity in the summed illumination slits of the fieldfacets.
 11. A method according to claim 8, wherein the partial fieldfacets comprise a pair of identical partial field facets configured tobe symmetrical in a pupil plane of the pupil faceted mirror.
 12. Amethod according to claim 8, further comprising second blades configuredto block radiation from the radiation source, each second blade beingselectively actuable to cover a portion of a selectable number ofpartial field facets.